1. Field of the Invention
The present invention relates to a sheet processing apparatus that performs post-processing on sheets having images formed thereon.
2. Description of the Related Art
In recent years, as an optional device that is connected to an image forming apparatus, such as a laser beam printer, there has been proposed a sheet processing apparatus that performs post-processing, such as a sorting process for sorting sheets having images formed thereon, and a stapling process for stapling a plurality of sheets having images formed thereon into one bundle.
When the stapling process is performed, it is required for the sheet processing apparatus to complete a process for aligning a set of sheets to be stapled and stapling the aligned set of sheets into a bundle within a time period corresponding to a time interval between when the last sheet of one set of sheets is conveyed and when the first sheet of a next set of sheets is conveyed. However, if the time interval of conveyance between the above-mentioned two sheets is short, the first sheet of the next set is sometimes conveyed in during execution of the process for stapling the sheet bundle of the one set, which prevents the process from being completed.
To cope with such a problem, there has been proposed a sheet processing apparatus that, during performing the process for stapling a sheet bundle of one set of sheets, causes first several sheets of a next set of sheets to be temporarily held back in a buffer path, thereby securing a time period for completing the process for stapling the sheet bundle of the one set of sheets.
For example, there has been proposed a method of securing a time period for completing the process for stapling a sheet bundle of one set of sheets, by wrapping sheets around a buffer roller as a buffer path (U.S. Pat. No. 7,413,178).
In this method, a first sheet S1 of one set is wrapped around the buffer roller. Thereafter, a sheet S2 conveyed following the sheet S1 at a predetermined timing is wrapped on the sheet S1, and the two sheets are conveyed as one sheet bundle. At this time, the sheets S1 and S2 are superposed one upon the other in an offset state (displaced state) in the conveying direction. At this time, the sheets S1 and S2 are offset such that a leading end of the sheet S2 protrudes from a leading end of the sheet S1 (protrudes downstream in the conveying direction). This offset is necessary for properly aligning the superposed sheets S1 and S2 on an intermediate processing tray. Further, a displacement amount in the conveying direction between the sheets S1 and S2 superposed in the offset manner, i.e. an offset amount is determined depending on timing in which the sheet S1 is sent out.
Then, the sheet bundle formed by the superposed sheets S1 and S2 is discharged onto the intermediate processing tray, and is aligned thereon.
A description will be given of how a sheet bundle formed by a plurality of superposed sheets is aligned, with reference to FIG. 12. FIG. 12 schematically shows the sheet bundle formed by the superposed sheets in a state discharged onto the intermediate processing tray.
As shown in FIG. 12, the sheet bundle formed by the superposed sheets S1 and S2 is discharged onto the intermediate processing tray 138 by a lower conveying roller 128. At this time, the sheet bundle is conveyed with the leading end thereof nipped between a bundle discharge roller pair 130. Then, when the sheet bundle passes through the lower conveying roller 128 and is placed on the intermediate processing tray 138, the rotation of the bundle discharge roller pair 130 is reversed, whereby the sheet bundle is conveyed toward a rear end stopper 138a disposed on the intermediate processing tray 138.
Thereafter, the bundle discharge roller pair 130 is separated from each other, whereby each of the sheets S1 and S2 of the sheet bundle is moved by inertia toward the rear end stopper 138a, which brings the rear end thereof into abutment with the rear end stopper 138a. At the start of this downward movement of each of the sheets S1 and S2, the sheet S1 under the sheet S2 is in a state offset from the sheet S2 such that the trailing end of the sheet S1 is closer to the rear end stopper 138a than that of the sheet S2 is. Therefore, the trailing end of the sheet S1 is the first to be brought into abutment with the rear end stopper, and the trailing end of the sheet S2 is the next.
As described above, the sheets S1 and S2 are offset such that the rear ends thereof are sequentially brought into abutment with the rear stopper, from the sheet S1 in the lower position in the sheet bundle first, whereby the alignment of the sheets S1 and S2 in the conveying direction is properly performed.
Now, the above-mentioned offset amount is determined depending on whether or not the sheet bundle formed by the plurality of superposed sheets can be aligned on the intermediate processing tray, and is set to a fixed value (usually, not less than 1 mm).
Further, the number of sheets to be buffered, i.e. the number of sheets to have a sheet superposed thereon is determined depending on the length of a time period required to execute post-processing, the conveying capacity of a conveying path, etc.
However, when a sheet to be buffered and a following sheet are superposed one upon the other, there is a case where the offset amount in the conveying direction between the buffered sheet and the following sheet does not become equal to a predetermined offset amount. Presumably, this is caused by deflection of sheets in the conveying path in the conveying direction of the sheet due to a property (elasticity) of a material of the above-mentioned sheet, etc. In case a plurality of sheets forming a sheet bundle which are superposed as described above with a different offset amount from the predetermined offset amount are subjected to alignment on the intermediate processing tray, there is a case where it is impossible to reliably perform alignment of the sheets, causing misalignment.
To cope with this problem, a technique for always maintaining a fixed offset amount has been proposed (Japanese Laid-Open Patent Publication No. 2007-070079). According to this publication, the length of a sheet bundle formed by superposing a sheet S1 and a sheet S2 one upon the other on a buffer roller is measured, and if the measured length is longer than a predetermined length, the offset amount applied to superposition of sheets performed next time is reduced.
However, a sheet discharged from an image forming apparatus of an electrophotographic type is contracted when passing through a heat fixing device. Further, the amount of contraction of the sheet varies with the properties of a material of the sheets, the amount of toner transferred onto the sheet, the water content of the sheet, etc.
However, the technique proposed in Japanese Laid-Open Patent Publication No. 2007-070079 does not take the contraction of each sheet into account, which makes it impossible to accurately adjust the offset amount. As a result, there is a fear that it becomes impossible to accurately align sheets on the intermediate processing tray.